Such a pipeline guide assembly is often referred to as a “stinger”.
In the field of the art, pipeline laying vessels are provided which are based on J-lay, S-lay, or reel-lay. These pipelaying methods are widely known. Some other variants are also used.
In these known methods, a pipeline is laid by the vessel. As the vessel moves forward, the pipeline is launched from the vessel and lowered to the seabed. Many publications are known in the field of the art on this subject.
In certain conditions, the departure angle of the pipeline directly below the vessel can be different than the angle of the firing line of the pipe lay equipment on board the vessel. This is due to the fact that the natural departure angle directly below the vessel is determined by a number of factors, such as the water depth, the stiffness of the pipeline, the axial tension in the pipeline and other factors. The angle of the pipelay equipment may be determined by other factors, such as limitations of the vessel or limitations of the pipelay equipment or by weathervaning requirements of the vessel. These factors may lead to a difference in the natural departure angle and the pipelay angle, which is the angle between the vertical and the firing line along which the pipelay equipment deploys the pipeline.
Therefore, there is a general need in the field of the art to provide a pipeline guide assembly which guides the pipeline along a curved trajectory when it is launched from the vessel equipment, in order to provide a gradual transition from the pipelay angle to the natural departure angle of the pipeline. The pipeline guide assembly limits strains and/or stresses in the pipeline, preventing overstressing or even buckling of the pipeline.
In a J-lay system, the J-lay tower is normally positioned under an angle that generally corresponds to the natural departure angle under which the pipe string will leave the J-lay tower, the natural departure angle being a function of the pipe properties, water depth and horizontal force which is exerted on the pipe by the pipelay vessel. The advantage of this method is that the pipeline does not require much support after leaving the pipelay tower, the pipe laying process does not lead to (much) additional stress and strain in the pipeline, and the horizontal force exerted on the pipeline by the pipelay vessel is small when compared with for instance the S-lay method. This offers the benefits that positioning of the pipeline on the seabed can be done more accurately, tighter curves can be made and freespans are significantly reduced.
Therefore, in a J-lay system a relatively short and thus relatively small support structure will suffice, which may be used for purposes of monitoring rather than supporting the pipeline. A disadvantage of this method is that the pipelay vessel needs to be positioned such that the J-lay tower firing line is always lined up with the pipelay route. The heading of the vessel can thus not be optimized to suit local circumstances like waves or current.
In a J-lay system, it is also possible to maintain the J-lay tower in a generally vertical position, wherein the pipe is supported between the vertical position in the tower and the natural departure angle below the tower. With this method, also known as Vertical lay or V-lay, the heading of the pipelay vessel is no longer dictated by the pipeline route, but can be chosen such that the vessel propulsion and station keeping system can be utilized in the most suitable way for encountered circumstances, for instance waves or current.
A disadvantage of this method is that the pipeline needs to be supported over a larger distance in order to bridge the angle between the vertical construction angle and the natural departure angle. Further a strong support structure is required since the horizontal component of the pipeline lay tension is transferred between the pipeline and the vessel via the stinger roller boxes instead of via the axial pipe direction through the pipe hang off system on the vessel which is the case for ‘normal’ J-lay.
Since the vessel can be oriented in any position, the support structure needs to be able to support the pipe in any direction over a considerable length. This may potentially lead to a very large structure.